For example, as an autofocus (hereinafter, abbreviated as “AF”) method of an image capturing device such as a digital (video/still) camera, a phase detection AF (hereinafter, also referred to as “PDAF”) method, a contrast detection AF (hereinafter, also referred to as “CDAF”) method, and a hybrid AF method in which PDAF and CDAF are combined are known.
In the phase detection AF method, AF is performed by using a phase difference between two images obtained by splitting an exit pupil of an imaging optical system.
As the phase detection AF method, there is a method in which an external phase detection sensor is provided in addition to an image sensor serving as an imaging element for taking an image and a phase difference is obtained on the basis of output of the phase detection sensor. Further, as the phase detection AF method, there is an focal plane phase detection AF method in which detection pixels for detecting a phase difference, which receive light passed through different regions of an exit pupil of an imaging optical system, are arranged as a part of pixels included in an imaging element and a phase difference is obtained on the basis of pixel values of the detection pixels.
In the phase detection AF method, AF scanning operation for moving a focus position (lens position) of the imaging optical system does not need to be performed at the time of AF, and therefore it is possible to execute AF for a comparatively short time.
Meanwhile, in the contrast detection AF method, a contrast evaluation value for evaluating a contrast of an image taken by an image sensor is obtained while AF scanning operation is being performed. Then, a lens position of an imaging optical system at which the contrast evaluation value is the largest is detected as a focusing position, and the lens position is moved to the focusing position.
In the contrast detection AF method, detection accuracy of the focusing position is generally high, as compared to the phase detection AF method. However, it takes time to execute (complete) AF in some cases because the AF scanning operation is performed.
The phase detection AF method is excellent in terms of execution time of AF (detection time of the focusing position), and the contrast detection AF method is excellent in terms of accuracy of AF (detection accuracy of the focusing position). The hybrid AF method has both excellent points of the phase detection AF method and the contrast detection AF method.
That is, in the hybrid method, a lens position (of an imaging optical system) is first moved to a position near a focusing position by the phase detection AF method, and then the lens position is moved to the focusing position by the contrast detection AF method. With this, it is possible to move the lens position to the focusing position for a short time with high accuracy.
Further, in recent years, there has been proposed a function referred to as so-called continuous AF that continuously performs focusing on a subject without fixing a focus position. In the continuous AF, the focus position is not fixed, and therefore it is effective in performing focusing on a moving subject in particular, and the continuous AF may also be applied in a case where a moving image is captured. For example, Patent Literature 1 discloses an example of the continuous AF.